Add support for processing .eh_frame.

This adds support for:
* Uniquing CIEs
* Dropping FDEs that point to dropped sections

It drops 657 488 bytes from the .eh_frame of a Release+Asserts clang.

The link time impact is smallish. Linking clang with a Release+Asserts
lld goes from 0.488064805 seconds to 0.504763060 seconds (1.034 X slower).

llvm-svn: 252790